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kx500 engine in a 05-07 kx250 2 stroke frame


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Hi everyone,

I've got a kx500 that goes hard but my suspension, wheels etc are getting real tired and need some work. Instead of spending a heap of cash on fixing everything up and still having some old tech chassis, I'm considering putting the motor into a late 250 2 stroke frame 2005-2007(not Al). There have been a few of these conversions done but the latest one I have seen done is on an 04 frame. Even though the frames changed from 05 onwards, I'm sure the motor will still fit. My question is that since the 05 and later frames have their rake reduced from 27 degrees to 26 degrees, I'm wondering how this will affect high speed stability. I haven't ridden a late 250 2 stroke so I have no idea. My bike is geared for around 100mph and I was wondering how stable the late bikes would be at these speeds as I do a lot of desert riding. I would hate to spend the time and money on a conversion and find that the bike is unstable at high speeds. If anyone owns one of the later 250's I would appreciate if you could give me an idea of how stable they are at high speeds and if the 26 degree rake has lessened the stability over the old bikes with the 27 degree rake.

Sorry for such a long winded question but I have been keeping a lookout for some late 250's and a couple of options have finally come up. Just don't want to make a bad mistake.

Thanks

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Well Keep in mind, You can put different triple trees on really easily, to change the characteristics. I can tell you dont believe in high dollar stabilizers, neither do I really as none of bikes have them... That would fix the problem though. If the front end isnt right it wouldnt be hard to find some different offset trees to fix this.

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Thanks for that. I've read a couple of magazine tests that got me confused. The tests on the 05 said it handled great but the tests I read on the 06-07 said it was a flawed design as the back of the bike sat up too high. I'm not even sure if the 05 has the same basic suspension setup as the 06 & 07 and is also too high at the back. Would love to ride one just to see how stable they really are. That said, it sounds like I've got some options if stability is an issue

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I have a 03,04 and 07. They are setup for woods racing. The 07 is my fiances and neither I or her have noticed any wierd handling traits with it. It turns as well as the 03 and 04. No headshake, although it does have a steering dampener. Im building a 04 KX250/500 but was wondering if the 500 engine would fit the newer 05 up chassis also. Looking at them side by side, it looks like it would be about the same amount of work, which isnt much, to put the 500 into the newer chassis.

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Well we dont live in the desert but the enduros that we race have high speed fire breaks, basically a trench dug through the woods by a machine to help keep brush fires from spreading and they are usually pretty straight which lends themselves to very high speeds. There are high speed trails, again not 100 mph but pretty darn fast for NJ and we both havent had any headshake problems. Most of the trail is very tight, where if there was a handling issue it would surface there also. All our bikes have been revalved and that also could be a contributing factor to their stability. Both of us are A level riders so we do push the bikes pretty hard.

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Not all of my riding is at those speeds, most of it is at what you would consider normal speeds. I suppose that as long as I get my suspension sorted and maybe a few other mods, It should make a good fun package. It will be nice having the power as well as something that handles well. Thanks for the help guys.

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The kx500 has a trail of 4.6in and the kx250 has a trail of 4.1in. I don't really understand exactly how this affects handling or whether the difference between the two would make a big difference. I do know that my 500 is very stable at speed.

Not sure if the earlier models are an easier conversion but I think that they have the head stay at the front whereas the later ones have it at the back which is the same as the kx500. I know that people have done conversions on both. I'm sure each would have their own issues.

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No matter what you'll be fabricating and welding. The big key is what I posted earlier about the S/A bolt. That locates the engine, doesn't matter where the headstay or other engine mounts are, if there's an issue with that s/a bolt you'll have a ton of work ahead of you. A 500 in my 05 chassis would be sick...who cares about rake/trail, the chassis is awesome.

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No matter what you'll be fabricating and welding. The big key is what I posted earlier about the S/A bolt. That locates the engine, doesn't matter where the headstay or other engine mounts are, if there's an issue with that s/a bolt you'll have a ton of work ahead of you. A 500 in my 05 chassis would be sick...who cares about rake/trail, the chassis is awesome.

It is widely known when doing the 500 conversion to a newer chassis that the cases have to be milled out at the swingarm pivot from 15 to 17mm. In addition to that you will need to cut the swingarm and spacers about 9mm if I remember to fit around the wider 500 cases. On the KX250 the lower engine case mount works, but you will need to fab a new front case mounts and headstay.

I'm working an 03 conversion. The 03-04 are pretty impossible to fit an oversized tank to, stock works though. I believe it was zz3 that posted up on kxriders that the 05+ oversized tank should work on the 03-04 with slight mods. I would assume that they would offer the same clearance on the 05+ frame.

The newer 03+ chassis is a lot better than the old, and I prefer the stock style rear headstay.

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I've read that the '02 and earlier frames make for an easier 500 conversion, but I don't have personal experience.

The only thing that would be different would be the headstay, and since you are already welding mounts on the frame that is minor.

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That was me that said that the 05 up Clarke tank fits pretty good. The only issue is the rear headstay on the frame. It touches the tank and I cant get any bolts in to the headstay plates. I have to cut the mount and line it up with the head mount. Ive been trying to get a friend to do the needed welding so I can wrap this project up but he has been too busy lately. I may go another route just to get it done.

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